1. If i buy a pre-built (i LOOOOVE the Sinister Prophecy), will i be able to change out the parts in future with parts from the ASP store? ie are pre-built sabers ASP compatible?
Yes... and no. Most pommels will be easy to switch out but some pre-built hilts are configured differently than their ASP counterparts. The most clear example of this is the Epoch which has its switch section in different parts of the hilt depending on whether it's the ASP or pre-built model. The pre-built parts are also glued together so if you did want to play around with swapping parts out you'd need to request the components not be glued. Some hilts will never make the jump to ASP if their design can't be split into the ASP configuration of pommel/body/switch/emitter.
2. Looking at the two options, pre-built vs ASP, it seems like you can build almost any combination on ASP (again i use sinister prophecy as example) that is available prebuilt at a significantly cheaper price, and if i am reading it correctly, ASP delivers in February while prebuilt needs 14-16 weeks bringing delivery time to March/April. Is there a significant reason why I should be going for prebuilts vs ASP? Am i missing something here?
Part of the ambiguity you mention comes from wait times being a bit askew due to a variety of reasons (holidays, TFA, ASP launch, etc.). Prices, though, are a smidge cheaper and you get the stands/belt clips as extras. I
suspect that the pre-built v. ASP difference will be largely aesthetic as the pre-builts and ASP sabers will look slightly different from each other due to the considerations taken to break the design up into the four parts.
3. What is the optimal length for a saber hilt? As mentioned I love the Sinister Prophecy but I also really like the idea of a leather wrapped hilt.. playing around with the ASP, i get a custom hilt that is about 15inches long. Is that too big? I'm not very tall, only about 5"6 so I worry if this becomes unwieldy. Any advice?
My preferences skew towards longer is better. It entirely depends on your dueling style. Also just because a saber is long that doesn't mean that all of the length is space your hands can grip.
*eyeballs your design*
For a hilt that long a two handed grip would probably be the most natural way to wield it. I would wager that one handed spinning would be a bit difficult since it is so long. *shrugs* Whether a hilt is too big or small is really up to the personal preferences of the wielder. Personally I favor longer hilts because I feel they stronger more intentional duelers vs. the one handed hilts.